1) someone or something that just isnt right.
2) the feeling you get the morning after usuing a lot of drugs, most commonly associated with extacy.
3) something unsafe
4) someone or something that gives off a bad feeling

Sara is kind of a sketchy person to hang out with because she gets drunk a lot and she tends to get other people in trouble.
The directions on this worksheet are sketchy. I do not think that they are giving the correct information on how to multiply fractions.

creepy, iffy, fairly unsafe, an air of uncertainty, not kosher, and just generally something or someone that you don't want to be associated with (or really do want to be associated with, depending on who you are...)

This expression was first coined in reference to hastily drawn out, or sketched, building/construction plans, which were uncertain at best and disastrous at worst. The overall thread of meaning is the same, but now applies to morality/legality. On the overall continuim of goodness, things progress, in order: safe, sketchy, shady, outright criminal.

Getting wasted and gambling away half your paycheck in Nevada may be legal, but its definitely sketchy.